Re: [PATCH v2 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for LLVM=1 LLVM_IAS=1

From: ebiederm@xmission.com (Eric W. Biederman)
To: Masahiro Yamada <masahiroy@kernel.org>
Cc: Arnd Bergmann <arnd@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Miguel Ojeda <ojeda@kernel.org>,
	Fangrui Song <maskray@google.com>,
	Michal Marek <michal.lkml@markovi.net>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Christoph Hellwig <hch@infradead.org>,
	Nathan Chancellor <nathan@kernel.org>
Subject: Re: [PATCH v2 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for LLVM=1 LLVM_IAS=1
Date: Mon, 26 Jul 2021 15:27:02 -0500	[thread overview]
Message-ID: <87r1fkizxl.fsf@disp2133> (raw)
In-Reply-To: <CAK7LNATLy2F-2zkHm4ENSufBT_o5p=9jc5k1K-xOV8cQf7kKDw@mail.gmail.com> (Masahiro Yamada's message of "Wed, 21 Jul 2021 13:04:16 +0900")

Masahiro Yamada <masahiroy@kernel.org> writes:

> On Wed, Jul 21, 2021 at 4:58 AM Arnd Bergmann <arnd@kernel.org> wrote:
>>
>> On Tue, Jul 20, 2021 at 7:43 PM Linus Torvalds
>> <torvalds@linux-foundation.org> wrote:
>> > On Tue, Jul 20, 2021 at 1:05 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
>> >
>> > We do most of the other heavy lifting in this area in Kconfig anyway,
>> > why not add that compiler choice?
>> >
>> > Obviously it would be gated by the tests to see which compilers are
>> > _installed_ (and that they are valid versions), so that it doesn't ask
>> > stupid things ("do you want gcc or clang" when only one of them is
>> > installed and/or viable).
>>
>> I don't see a good way of making Kconfig options both select the
>> compiler and defining variables based on the compiler, since that
>> would mean teaching Kconfig about re-evaluating all compiler
>> dependent settings whenever the first option changes.
>>
>> I do have another idea that I think would work though.
>>
>> > Hmm? So then any "LLVM=1" thing would be about the "make config"
>> > stage, not the actual build stage.
>> >
>> > (It has annoyed me for years that if you want to cross-compile, you
>> > first have to do "make ARCH=xyz config" and then remember to do "make
>> > ARCH=xyz" for the build too, but I cross-compile so seldom that I've
>> > never really cared).
>>
>> The best thing that I have come up with is a pre-configure step, where
>> an object tree gets seeded with a makefile fragment that gets included
>> for any 'make' invocation. This would set 'ARCH=', 'CROSS_COMPILE',
>> 'CC=' and possibly any other option that gets passed to 'make' as
>> a variable and has to exist before calling 'make *config'.
>
>
> There is no need to add a hook to include such makefile fragment(s).
>
> Quite opposite, you can put your Makefile (in a different filename)
> that includes the top Makefile.
>
>
> I think this is what people are already doing:
>
>
> GNU Make looks for 'GNUmakefile', 'makefile', and 'Makefile'
> in this order.
>
>
> So, you can put 'GNUmakefile' with your favorite setups.
>
>
> $ cat GNUmakefile
> ARCH=arm64
> CROSS_COMPILE=aarch64-linux-gnu-
> CC=clang
> include Makefile

Very weird.

I just tested this and it does not work.
I did this:

$ cat GNUmakefile
ARCH = alpha
CROSS_COMPILE = $(arch-prefix alpha)
include Makefile

In one of my build directories and the main makefile simply does not see
the value of ARCH or CROSS_COMPILE I set.  I have confirmed that my
GNUmakefile is being read, because everything breaks if I remove the
include line.

Does anyone have any ideas?

Something so we don't have to specify all of these variables on the make
command line would be nice.

Eric